Various telescopic systems arranged so as to inform direction data while scene or the like is being observed have been proposed from the standpoint of improvement of convenience thereof for example. For example, telescopic systems comprising a magnet plate provided in a liquid such as oil arranged such that the magnet plate can be viewed in the field of view using an optical system have been known (JP-A-2001-13420).
Further, telescopic systems capable of displaying direction data obtained by a magnetic sensor in the field of view and informing the angular deviation from the line of sight detected by a tilt sensor provided therein have been known (JP-A-2002-221670).
However, the former of the aforementioned related art examples, i.e., structure having a magnet plate disposed in a liquid is disadvantageous in that when the binocular is tilted with respect to the horizontal position, the magnet plate, too, is tilted according to the deviation, making it difficult for letters indicating direction to be viewed as compared with the case where the magnet plate is viewed from the front.
This problem will be further described hereinafter in connection with FIGS. 10 and 11 with reference to a binocular having a compass provided with a magnetic plate similarly to the binocular disclosed in JP-A-2001-13420, though somewhat different in structure, by way of example. The compass portion of the binocular shown in FIG. 10 has a magnet plate 51 received in a liquid such as oil as in the binocular disclosed in the above cited patents. The magnet plate 51 is disposed on the optical axis L extending between a relay lens 52 for introducing light flux into the ocular of the main body of the binocular and a light window 53. In this arrangement, the direction indicated on the magnet plate 51 can be viewed while observing scene, etc. FIG. 10 depicts the state in which the binocular is kept horizontal and the optical axis L is kept vertical. The magnet plate 51 is disposed along the horizontal direction. In this state, the magnet plate 51 can be viewed from the front in the field of view, making it possible to read the direction without being out of focus.
On the other hand, when the binocular is tilted with respect to the horizontal direction and the optical axis L is tilted with respect to the vertical direction as shown in FIG. 11, the magnet plate 51 in the liquid changes in its position such that it is disposed along the horizontal direction. As a result, the magnet plate 51 is tilted with respect to the direction perpendicular to the optical axis L. This causes the magnet plate 51 to be viewed obliquely. Therefore, the direction must be viewed out of focus, making it difficult to read accurate direction immediately to disadvantage.
On the contrary, the related art binocular disclosed in the above cited JP-A-2002-221670 comprising a magnetic sensor is adapted to electrically process the output signal from the magnetic sensor and display it in the field of view. Thus, this system doesn't cause the display to be viewed out of focus as in the former citation. However, since this magnetic sensor, too, requires that it be used while being kept horizontal, some error in direction occurs when the binocular is tilted with respect to the horizontal direction. Such erroneous direction data need to be corrected.
In order to correct the erroneous direction data, the tilt of the binocular with respect to the horizontal position needs to be known. To this end, a tilt sensor is needed. At the same time, it is necessary that the structure for mounting the tilt sensor be taken in account. This complicates the structure of the binocular. Further, the complicated correction process adds to product price.